1. Field of the Invention
The present invention generally concerns ceramic filter capacitors such as are most commonly discoidal in form.
The present invention particularly concerns the organization of internal electrodes within a discoidal feed-through ceramic filter capacitor so as to, by use of the electrodes"" fringing fields, realize a compact capacitor of very high breakdown voltage.
2. Description of the Prior Art
The present invention will be seen to concern the size and placement of the internal electrodes within a multi-layer feed-through ceramic filter capacitor. Such a capacitor is commonly discoidal in shape, passing wires upon which are carried electrical signals to be filtered through one or more holes that are most commonly aligned to the axis of the disc. (Other geometries are, however, possible. See, for example, U.S. Pat. No. 5,822,174.) Typically multiple metal plate regions of each of a first, and of a second, electrode are typically located in partially overlapping positions upon alternating ceramic layers, thereby to create capacitance between these interleaved metal plate regions, and between the electrodes that are formed from the plate regions.
Capacitors so constructed are of particular use to filter electrical signals upon the wires and leads of implanted cardiac pacemakers and cardiac defibrillators. These latter devices use high voltages, commonly about 750 volts.
When the electrodes of a filter capacitor are subjected to high voltagesxe2x80x94on the order of hundreds and, with safety margins, even thousands of voltsxe2x80x94then their partially-overlapped metal plate regions are subject to develop voltage breakdown paths (1) through the ceramic to each other, and/or (ii) to oppositely-charged regions of the outside surface of the capacitor where electrical connections are madexe2x80x94all as is more thoroughly discussed in the DESCRIPTION OF THE PREFERRED EMBODIMENT section of this specification. According to this propensity, the alternating polarity metal plate regions of the two electrodes must be separated both (i) from each other, and (ii) from oppositely-charged regions at the exterior of the capacitor, by a reasonable thickness of ceramic.
Because (i) capacitance between spaced-parallel plate regions is a function of their separation, and (ii) plate density cannot be particularly high in a multi-layer capacitor where only an adequately thick ceramic buffer can insure such a high breakdown voltage as is desired, it has heretofore been believed that the metal plate regions of alternating polarity should be positioned (i) spaced-parallel (ii) in at least partial overlap along the axis of the discoidally-shaped ceramic body and of any hole(s) in this body. The simple reason that the metal plate regions are so located overlapping spaced parallel is so as to create capacitance along the elementary model of two parallel plate electrodes. Indeed, the formula for the capacitance of the conventional parallel-plate ceramic capacitor is:   Cap  =      kA    d  
where Cap is the capacitance in farads, k is the dielectric constant in farads per meter, A is the area of electrode overlap in square meters and d is the distance of separation between plates in meters.
The present invention will be seen to show that capacitance suitable for filtering can be realized by an alternative metal plate configuration; a configuration that is much improved for voltage breakdown. A hallmark of this configuration, and of the present invention, will be seen to be the plate regions of each first and each second electrode pair (being that some filter capacitors can contain multiple electrodes) will be non-overlapping along the axis of the discoidally-shaped ceramic body, and along the axis of any hole(s) in this body.
Both the overlapping electrode plate regions of prior art multi-layer ceramic filter capacitors, and the multi-layer ceramic filter capacitors of the present invention totally lacking any such overlap in the, are highly visually distinctive (in views of the capacitor interiors). The overlapping of electrode plate regions is not much commented upon in the prior art, most likely because such has previously been deemed a fundamental and immutable basis of the construction of ceramic capacitors of all typesxe2x80x94including feed-through filter capacitors. Accordingly, the next-following written descriptions of selected prior art patents, while amply showing the state of the art in multi-layer ceramic feed-through filter capacitors, do not deal directly with the present invention. The figures of these prior art patents do, however, clearly show that overlap of electrode plate regions that is obviated by the present invention.
For example, U.S. Pat. No. 4,148,003 to Colburn, et al. for a SERIES FEED-THROUGH CAPACITOR concerns a feed-through capacitor of the type for use in filters and the like and including discoidal ceramic capacitors arranged in stacked relationship and electrically connected in series. Series arrangement of the discoidal capacitors prevents shorting through the feed-through capacitor in the event of failure of one of the discoidal capacitors therein and results in substantially improved insertion loss characteristics when compared to feed-through capacitors having a single discoidal capacitor. Internal electrode metal plate regions are overlapping.
U.S. Pat. No. 5,333,095 to Stevenson, et al. for a FEED-THROUGH FILTER CAPACITOR ASSEMBLY FOR HUMAN IMPLANT concerns a feed-through filter capacitor assembly and related installation method are provided for shielding a conductive terminal pin or lead of the type used, for example, in an implantable medical device such as a heart pacemaker or defibrillator, against passage of external interference signals. The feed-through assembly includes a terminal pin subassembly having at least one terminal pin supported within a cylindrical conductive ferrule by a hermetically sealed insulator ring. The ferrule is adapted for mounting onto a conductive pacemaker housing, as by welding or brazing, to support the terminal pin for feed-through passage to the housing interior. A co-axial filter capacitor is mounted at an inboard side of the pacemaker housing, with capacitor electrode plate sets coupled respectively to the pacemaker housing and to the terminal pin by a conductive adhesive or the like. In one preferred form, multiple filter capacitors are provided in a substantially co-planar array within a common base structure, with each capacitor in association with a respective terminal pin. Internal electrode metal plate regions are again overlapping.
U.S. Pat. No. 5,751,539 also to Stevenson, et al. for an EMI FILTER FOR HUMAN IMPLANTABLE HEART DEFIBRILLATORS AND PACEMAKERS concerns an improved ceramic feed-through capacitor design which results in distinct advantages in EMI (electromagnetic interference) filtering and therapeutic wave form management for implantable defibrillators and pacemakers and the like. The invention provides ceramic capacitor electrode plate designs which provide both low impedance de-coupling for EMI suppression, and, at the same time provide an isolated common ground point through a separate coupling capacitor for electrical isolation of the filtered circuit from the metal case (usually titanium) of the defibrillator or the like. Such an arrangement allows the defibrillator HV (high voltage) output pulse to the heart to be referenced lead to lead (including reverse polarity), or from either lead to a common floating ground point or to the titanium case. The primary application of the invention is directed to implantable defibrillators where the output pulse to the heart is typically high voltage (up to 750 volts) and of short duration (typically in the 10xe2x88x922 seconds, or 10 millisecond, range). The invention is equally applicable to implantable pacemakers. The invention is stated to be suited for a combination defibrillator (tachycardia) and pacemaker (bradycardia) unit where there are sense (heart monitoring), low voltage (pacing) and high voltage (defibrillator) output leads. Internal electrode metal plate regions are yet again overlapping.
U.S. Pat. No. 5,825,608 to Duva, et al. for a FEED-THROUGH FILTER CAPACITOR ASSEMBLY concerns a feed-through filter capacitor assembly is provided to pass a conductor into a housing of an electronic device while maintaining a hermetic seal and filtering spurious radio frequency signals. The assembly is particularly suitable for use in medical implant devices such as pacemakers. The feed-through filter capacitor assembly comprises a conductive bushing forming a cup that receives a filter capacitor body. The cup is located to one side of a pass-through portion including a passageway in which an insulating spacer is mounted. A plurality of wires pass through the filter capacitor body and spacer. The wires are contacted with inner plates of the capacitor and insulated from the bushing. Conductive polymer resin within the cup provides electrical continuity between outer contacts of the capacitor body and the conductive bushing. This arrangement also provides an effective heat sink for the filter capacitor body and protects the filter capacitor body from physical damage. The arrangement of a cup to the side of the pass-through portion allows sizing of the capacitor independently of the size of a housing hole provided to receive the assembly. Internal electrode metal plate regions are still yet again overlapping.
U.S. Pat. No. 5,999,398 to Maki, et al. for a FEED-THROUGH FILTER ASSEMBLY HAVING VARISTOR AND CAPACITOR STRUCTURE concerns a feed-through filter assembly such as may be used in an implantable medical device. The assembly includes a conductive mounting element which may be hermetically sealed to an outer housing of the implantable medical device. In many embodiments, the conductive mounting element will be a conductive canister in which a feed-through filter structure is located. Alternatively, the conductive mounting element may include a suitable sub-plate structure. Because the filter structure exhibits both varistor and capacitive characteristics, effective transient suppression and interference filtering is achieved in a single package. Secondary filtering may be provided downstream of the filter assembly for additional interference filtering at lower frequencies. Internal electrode metal plate regions are still yet again overlapping.
U.S. Pat. No. 6,008,980 again to Stevenson, et al. for a HERMETICALLY SEALED EMI FEED-THROUGH FILTER CAPACITOR FOR HUMAN IMPLANT AND OTHER APPLICATIONS concerns an integrated hermetically sealed feed-through capacitor filter assembly is provided for the shielding and de-coupling of a conductive terminal pin or lead of the type used, for example, in an implantable medical device such as a cardiac pacemaker or cardioverter defibrillator against passage of external interference signals, such as caused by digital cellular phones. The simplified feed-through assembly described herein eliminates the traditional terminal pin subassembly. In this novel approach, the ceramic feed-through capacitor itself forms a hermetic seal with a conductive pacemaker housing to which it is mounted by welding or brazing. The feed-through capacitor is configured such that its internal electrodes are not exposed to body fluids, with capacitor electrode plate sets coupled respectively to a conductive ferrule, pin or housing (which may be grounded) and to the non-grounded, or active, terminal pin(s) by conductive adhesive, soldering, brazing, welding or the like. In one preferred form, multiple feed-through filter capacitors are provided in a substantially coplanar array within a common base structure, with each capacitor in association with a respective terminal pin. Internal electrode metal plate regions are still yet again overlapping.
U.S. Pat. No. 5,905,627 to Brendel, et al. for an INTERNALLY GROUNDED FEED-THROUGH FILTER CAPACITOR concerns an internally grounded ceramic feed-through filter capacitor assembly that provides for the shielding and de-coupling of a conductive terminal pin or leadxe2x80x94of the type used, for example, in an implantable medical device such as a cardiac pacemaker or cardioverter defibrillatorxe2x80x94against passage of external interference signalsxe2x80x94such as those caused by digital cellular phones. The assembly includes a terminal pin subassembly having at least one terminal pin supported within a conductive ferrule by a hermetically sealed insulator structure. The ferrule is adapted for mounting onto a conductive substrate, such as a pacemaker housing, by welding or brazing to support the terminal pin subassembly for feed-through passage to the housing interior. A ceramic feed-through capacitor is mounted at an inboard side, with the capacitor electrode plate sets coupled respectively to a grounded lead and to the terminal pins(s) by conductive adhesive, soldering, brazing or the like. In preferred forms of the invention, multiple feed-through filter capacitors are provided in a substantially coplanar array within a common base structure, with each capacitor in association with a respective terminal pin. In this patent of Brendel, et al., the internal electrode metal plate regions are overlapping, as in all other patents.
The present invention contemplates organizing the internal electrodes within a feed-through ceramic filter capacitor, such as is commonly discoidal in shape, so as to, by use of the electrodes"" fringing fields, realize a filter capacitor that, nonetheless to being compact, has a very high breakdown voltage. Capacitors so constructed are of particular use in implanted cardiac pacemakers and especially, in consideration of the higher voltages used, implanted cardiac defibrillators.
A quality ceramic electromagnetic interference/radio frequency interference (EMI/RFI) filter capacitor in accordance with the most preferred embodiments of the present invention typically has a breakdown voltage reliably in excess of 4,000 volts, which is at least two, and more likely three, times better than the best capacitors of the prior art which are known to the inventors.
1. A Ceramic Filter Capacitor with Improved Internal Electrodes
In one of its aspects the present invention is embodied in a ceramic filter capacitor with internal electrodes improved in their size and placement so as to accord a much higher breakdown voltage to the capacitor than heretofore.
In some detail, the preferred filter capacitor is made from multiple ceramic layers as aggregate a substantially planar body. At least one hole in the body passes a wire that carries electrical signals which are to be filtered. A first metal area serves to line the body""s hole, and to make electrical contact with any wire within the hole. A second metal area serves as a band over at least part, and typically all, of an exterior edge surface of the body. This second metal band is suitably electrically connected to ground. So far the capacitor construction is conventional, and preferred filter capacitor of the present invention so constructed has a quite normal external appearance.
However, in accordance with the present invention a number of first internal metallization areas, each in the shape of a relatively-smaller small-aperture disk, are centered about the body""s hole each upon one of the body""s multiple ceramic layers. Each of these apertured disk metallization areas makes at its internal edge surface electrical contact with the metal lining of the hole, thereby to serve as a first electrode of the capacitor.
Meanwhile, a number of second internal metallization areas, each in the shape of a relatively-larger large-apertured disk, are centered about the body""s hole each upon one of the body""s multiple ceramic layers. Each of these second metallization areas makes at its exterior edge surface electrical contact with the second metal band, thereby to serve as a second electrode of the capacitor.
Importantly, the first internal metallization areas as collectively constitute the first electrode, and the second internal metallization areas as collectively constitute the second electrode, are separated from each other in radial displacement from an imaginary centerline of the body""s hole. This radial separation is typically large relative to both the minimum separations (i) between electrodes of opposite polarity, and (ii) between an internal electrode of one polarity and an external metal area of the other polarity, arising within discoidal feed-through EMI/RFI ceramic filter capacitors of the prior art. The larger separation between the electrodes of new geometry gives a higher breakdown voltage.
In user of this capacitor structure (i) upon electrical connection of the first metal to a wire passing through the hole, and (ii) upon electrical connection of the second metal to ground, a first fringing electrical field is developed at and between the first internal metallization areas, and a second fringing electrical field is developed at and between the second internal metallization areas. The metallization areas, and their fringing electrical fields, are internal within the ceramic body. A capacitance comparable with the capacitance of prior art feed-through capacitors is developed between these first and second fringing electrical fields. This capacitance is fully suitable to filter electromagnetic interference/radio frequency interference (EMI/RFI) present in electrical signals upon the wire.
The filter capacitor of the present invention is thus different not only in the sizes and the placement of its metallization areas, or electrodes, within the ceramic body, but by its intentional use, and coupling, of fringing fields as a primarily basis of a capacitance developed between the two electrodes. This is unusual: fringing fields are no longer secondary, but are rather primary, to capacitor performance.
The separation (i) between each two electrodes of a pair, and (ii) between any of the metallization areas as do collectively form one of the electrodes and those metallization areas as do collectively form the other one of the electrodes, is quite great relative to the separations arising between (i) the two electrodes, and (ii) the metallization areas, of previous ceramic filter capacitors. Accordingly, the breakdown voltage of a ceramic filter capacitor so constructed is already superior. However, in accordance with the present invention, the strength of the dielectric separation between opposite electrodes is still further improved.
Namely, the electrode metallization areas, or plates, are preferably located upon alternating layers. This makes an embodiment where any path of voltage breakdown will have to penetrate radially through, as well as transversely along, at least one ceramic layer. Voltage breakdown along such a path in twoxe2x80x94both radial and longitudinalxe2x80x94directions at the same time is strongly resisted, giving this embodiment of a multi-layer ceramic feed-through capacitor in accordance with the present invention an even higher breakdown voltage than the already impressively increased breakdown voltage of the basic embodiment of the invention.
2. A Ceramic Filter Capacitor Improved for the Strength of Dielectric Separation Between Electrodes
In another of its aspects the present invention may be considered to be embodied in a ceramic filter capacitor improved for the strength of dielectric separation between its internal electrodes. The material or nature of the capacitor dielectricxe2x80x94ceramicxe2x80x94is not changed: the size and placement of the metallization regionsxe2x80x94which regions in aggregate produce each of the electrodesxe2x80x94is instead improved. The metallization regions are sized and placed so as to make that any path by which the capacitor will suffer breakdown will have a much greater resistance (requiring a much higher voltage to induce breakdown) than heretofore.
The basic approach is (1) to separate the metallization areas widelyxe2x80x94as first described in section 1. abovexe2x80x94plus (2) to place each metallization region of each electrode on a separate ceramic layer from the metallization regions of the other electrode. The several metallization regions of each electrode are interleaved, with each metallization layer of each electrode being both (i) radially and (ii) longitudinally separated from the metallization areas of the other-electrode.
Accordingly, a ceramic filter capacitor so constructed has (1) its first internal metallization areas upon a first group of the multiple ceramic layers, (2) its second internal metallization areas upon a second group of the multiple ceramic layers, and (3) its first group of ceramic layers and first internal metallization areas interleaved with the second group of ceramic layers and second metallization areas. By this construction the first and the second metallization areas are never on the same ceramic layer, but are always separated, first metallization area to second metallization area, not only by the radial distance of separation, but also by a thickness of at least one ceramic layer.
Still further in accordance with the present invention, the ceramic filter capacitor may be constructed in diverse shapes, including in (i) the substantial shape of an apertured disk with one substantially central hole, or in (ii) the substantial shape of a multi-apertured substantially planar body with multiple holes each of which is capable of accepting at least one lead or wire. Regardless of the particular capacitor shape, the placement of the hole(s), such as by process of drilling, is not particularly dimensionally criticalxe2x80x94as it has been in the pastxe2x80x94because the holes simply pass into internal disk-shaped metallization areas (part of the first electrode) that are substantially larger than is the hole.
3. A Feed-Through Multi-layer Ceramic Filter Capacitor with Non-Overlapping Electrodes
According to the unique placement of the electrodes with feed-through multi-layer ceramic filter capacitors in accordance with the present invention, the invention may be broadly conceived to be embodied in a ceramic filter capacitor that is improved for the strength of dielectric separation between its internal electrodes.
It will be recognized that a common-form, prior art, multi-layer ceramic feed-through filter capacitor commonly has (1) a substantially monolithic body of layered ceramic dielectric material with at least one hole, suitable to accept an electrical conductor, passing through at least some layers of the body; (2) a number of conductive first electrode plates, encased within the body on an associated plurality of ceramic layers, electrically connectable to an electrical conductor within the hole; and (3) a number of conductive second electrode plates, encased within the body on an associated plurality of ceramic layers, electrically connectable to ground.
In this general structure the present invention is characterized in that the first plurality of conductive first electrode plates are spatially non-overlapping with the second plurality of conductive second electrode plates along any imaginary axis that is parallel to an axis of the hole.
These non-overlapping plates can be upon the same, or upon different, ceramic layers. Namely, and as a first alternative, at least some of the ceramic layers associated with the conductive first electrode plates can be the selfsame ceramic layers as are associated with the conductive second electrode plates. This makes that at least some of the conductive first, and the conductive second, electrode plates will be upon the same ceramic layers.
Alternatively, it can be that none of the ceramic layers associated with the conductive first electrode plates are the same as any ceramic layers associated with the conductive second electrode plates. This makes that at least one ceramic layer located between each conductive first, and each conductive second, electrode plate.
4. An Improvement to Discoidal Feed-Through Multi-Layer Ceramic Filter Capacitor
Still yet another way of describing the unique placement of the electrodes with feed-through multi-layer ceramic filter capacitors in accordance with the present invention is as follows.
The invention may be broadly conceived to be embodied in a feed-through filter capacitor having (1) a substantially discoidal dielectric body with (1a) a major axis, (1b) at least one hole, aligned parallel to the major axis, suitable to accept an electrical conductor, and (1c) an external rim region. The capacitor further has (2) a first set of electrodes, located spaced-parallel within the body transverse to the body""s axis, electrically connected to an electrical conductor within the hole, and (2) a second set of electrodes, also located spaced-parallel within the body transverse to the body""s axis, electrically connected to ground at the body""s rim. The electrical conductor is xe2x80x9cfed throughxe2x80x9d the body""s hole. This xe2x80x9cfeed-throughxe2x80x9d capacitor structure serves to filter to ground any electrical signal upon the electrical conductor fed through the body""s hole. All this is conventional.
To this structure the present invention constitutes an improvement wherein the first set of electrodes are non-overlapping with the second set of electrodes along any imaginary axis parallel to the axis of the at least one hole. This xe2x80x9cnon-overlappingxe2x80x9d simply means that no imaginary axis parallel to the axis of the at least one hole will pass through both an electrode of the first set and, also, and electrode of the second set.
The substantially discoidal dielectric body is preferably multi-layer ceramic, with the first and the second sets of electrodes being metal upon the ceramic layers.
As in section 3., above, the first and the second sets of electrodes may be upon either the same, or different, ones of the ceramic layers. Location of the electrodes on different, interleaved, layers is preferred.
These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification.